1. Field of the Invention
The present invention relates to a bidirectional communication system, more specifically, to a bidirectional communication system suitable for the use with a system in which a communication station is accessed by dial-up connection via analog telephone lines (PSTN: public switched telephone network) or integrated services digital network (ISDN), and exclusive (i.e., dedicated) lines for make use of services such as, for example, broadcasting and video on demand.
2. Description of Related Art
Recently, a new form of broadcasting services has become available by combining digital image compression techniques and a communication satellite communication (CS), as well as bidirectional communication.
The services include various kinds of multimedia services such as multi-channel broadcasting, video on demand, online-shopping, karaoke on demand, etc. The video on demand means a service providing a desired movie or a program at a specific time which a viewer desires, and in which the viewer has bi-directionality control in terms of video tape recorder (VTR) like control functions, e.g., pause, fast forward, and rewind. Karaoke on demand is similar to the video on demand, i.e., the delivery arrangement provides services of a karaoke program instead of movies and programs. These services are described in greater detail in Hiroshi Fujiwara, "Textbook Of Real MPEG", Multimedia Communication Workgroup Ed., ASCII Corp. (November 1995).
In furthering discussion, applicable digital image compression and communication satellite techniques will be described below, such techniques having been instrumental in having made available the above cited new form of services. More particularly, a typical image compression technique used in digital CS broadcasting is a MPEG-2 (Moving Picture Experts Group 2) technique. MPEG-2 was proposed as a standard in the ISO/IEC (International Standardization Organization/International Electrotechnical Commission), and such techniques makes possible image compression up to 4-9 Mbps with the image quality of current television broadcasting being held. Also, in standard digital CS broadcasting, a satellite transponder (transmitter responder) having bandwidth of 27 MHz is used for transmitting signals modified with Quadrature Phase Shift Keying (QPSK). By using QPSK modification, a signal of 54 Mbps may be transmittable, however, a substantial transmission rate per transponder may be approximately 27 Mbps except for redundancy codes added for error correction.
Thus, by using the above mentioned MPEG-2,3-6 programs may be transmitted per transponder to realize multi-channel broadcasting of several times, i.e., such transmission increase is advantageous as compared with the analog CS broadcasting in which only one program is transmitted per transponder. As a result thereof, the above cited new form of services may have been provided.
Most of these services are interactive services including bidirectional communication. Thus, not only are downstream links (a.k.a., down-links) from the communication station providing services to the viewers required, but also upstream links (a.k.a., up-links) from the viewers to the communication station are required. Currently, already existing analog phone lines (PSTN: public switched telephone networks), ISDN networks and exclusive lines work well for upstream links.
A background bidirectional communication system (suitable for background discussion with respect to the present invention) is comprised of a central communication station providing services, and remote viewers' terminals accepting services. More particularly, each viewer terminal is connected to the communication station via an upstream link such as an analog phone line, ISDN network or an exclusive line, and various broadcasting programs and multimedia services are received through a communication satellite link as a downstream link from the central communication station. Dial-up connection is usually used for connecting a viewer terminal to the communication station.
Returning now to discussion concerning providing/receiving broadcast/multimedia services, when making a dial-up connection from a viewer's terminal to a communication station through use of a phone line, at the time when the viewer's terminal dials the communication station through use of a phone line, the link is connected to a dial-up router of the communication station through the phone line network. Then, connection is made between the dial-up router and the viewer's terminal typically with a point-to-point protocol (PPP). PPP is a protocol for connecting between two points, e.g., a central communication station and a remote user terminal. For connecting with PPP, a data link layer of an open system interconnection (OSI) seven layered model is set for the remote communication station, thereafter the dial-up connected user is authenticated, and then the protocol for the network layer is set.
Dial-up connection is also frequently used for internet connecting, i.e., for connecting personal computer users (e.g., from home) to an internet provider, which, in general, is conducted via a PPP approach wherein a user initiates dial-up and log-on when the user wants to connect a personal computer to an Internet provider or Internet. More particularly, one typical example of the network layer protocol that the PPP sets at this time is an Internet Protocol (IP). The protocol of PPP for setting IP is Internet Protocol Control Protocol (IPCP). IPCP determines an IP addresses of both connected sides.
There are three methods for determining IP addresses as follows:
(1) assigning an IP address for each phone line number used for connection; PA1 (2) assigning a fixed IP address for each partner at the other side of the connection, by identifying the connected partner at the stage of authentication; and PA1 (3) assigning dynamically one of unused IP addresses from a pool of IP addresses.
Method (2) is a static assignment, whereas method (3) is a dynamic assignment. Method (1) may be a static assignment when connecting by directly specifying the line number, while it may be a dynamic assignment if the call is automatically (i.e., randomly) assigned to a plurality of lines available when a main number is called.
If an IP address is decided, the user may receive various multimedia services by using an appropriate protocol for transport layer (such as a Transmission Control Protocol (TCP), and User Datagram Protocol (UDP)).
Other system architectures other than a communication satellite of use with the present invention is a cable television system using a Hybrid Fiber Coax (HFC) network in which a main backbone network is constructed with optical fibers, and a secondary distribution network for connection from the HFC network to homes are constructed with coaxial cables. In the HFC network, upstream and downstream (i.e., bidirectional) links are both able to be set therewithin. However, there are cases where sufficient bidirectional quality may not be achieved if the condition of the transmission installation is not suitable, or where upstream links are not available because of limited bandwidth capacity and the fact that bandwidth is reserved for the upstream links. Accordingly, an analog phone network, ISDN network, and exclusive lines are projected to be used as main upstream links in the present invention.
Detailed description of the dial-up connection as described above may be found in the article "PPP and Dial-up IP Connection", Shunji Ohno, UNIX Magazine, February 1995, pp. 33-34, ASCII Corp.; detailed description of PPP may be found in RFC (Request For Comment) 1661, "The Point-to-Point Protocol (PPP)", W. Simpson Ed., published by Internet Engineering Task Force (IETF), July 1994; the outline of PPP may be found in the foregoing "PPP and Dial-up IP Connection"; description of IP may be found in "Internet Protocol", J. Postel Ed., September 1981, published by IETF; and, detailed description of IPCP may be found in "The PPP Internet Protocol Control Protocol (IPCP)", G. McGregor, May 1992, published by IETF.
When viewing a movie using a video-on-demand service, after the viewer selects a program, typically only a few video operation control commands are used (e.g., End command). Accordingly, at most, only several commands are output at a beginning and end of a program and possibly at some intermediate time during the program. If a communication link is held for one or two hours during program viewing in order to send these few commands, link usage is inefficient. In the method of assigning IP addresses in the existing Personal Computer Communication Systems and the Internet, an approach is useable to disconnect a link in a case where there is no communication within a predetermined period of time, i.e., in order to avoid ineffective use (holding) of IP addresses and lines.
However, if an IP address is dynamically assigned and the link is disconnected, it may be possible that a different IP address will be assigned for each dial-up call by a user. Since application programs such as control programs for video on demand may execute, for example, the selection of the source of command and the program to be affected by a command in accordance with the IP address assigned to the terminal of the viewer, there are problems that if the IP address changes by reconnection, the service may not be controlled correctly thereafter.
In a case where IP addresses are statically assigned, after disconnecting and subsequently attempting a new dial-up connection at a later time, a connection may not be able to be made because all lines to the dial-up router are busy. Further, in case of dynamic assignment, there may be a temporary lack of IP addresses to be assigned. As the terminal cannot recognize a lack of available IP addresses before dial-up, there are problems that a telephone line charge must be paid for because no IP address is obtained even if the link is connected to the dial-up router, and a user becomes frustrated. More particularly, in case of both dynamic and static assignment, when any disorder happens at the communication station, if a viewer's terminal attempts a dial-up connection thereto without knowledge of a present status of the station, the link may obtain a useless connection to the station because even if connected there will be unsuccessful PPP negotiation. In this case, a telephone company line charge must be unneedlessly paid because the phone line is used.